1. Field of the Invention
The present invention relates to a color splitting/combining optical system used for an image projection apparatus (projector) for projecting light from image-forming elements which form an original picture.
2. Description of Related Art
An image projection apparatus in which reflection type liquid crystal display elements and a polarization beam splitter are combined with each other is disclosed by Japanese Patent Application Laid-open No. 2001-154268. The corresponding image projection apparatus is an image projection apparatus having a white-color light source 1001, reflection type liquid crystal display elements 1002R, 1002G and 1002B, and a projection optical system 1003 as shown in FIG. 33. In the corresponding image projection apparatus, a dichroic mirror 1004 is provided between the white-color light source 1001 and the reflection type liquid crystal display elements 1002R, 1002G and 1002B. Further, the apparatus includes a color splitting system in which polarization beam splitters 1005 and 1006 are provided between the dichroic mirror 1004 and the reflection type liquid crystal display elements 1002R, 1002G and 1002B, and a color combining system in which the first, second and third polarization beam splitters 1005, 1006 and 1007 are provided between the reflection type liquid crystal display elements and the projection optical system.
Herein, the first color-selective wave plate 1008 capable of rotating the polarization direction of light of a predetermined wavelength region by 90 degrees is provided between the dichroic mirror 1004 and the second polarization beam splitter 1006, and the second color-selective wave plate 1009 is provided between the second polarization beam splitter 1006 and the third polarization beam splitter 1007, wherein color components (R and B) and polarization directions (P and S) are associated with each other, and color splitting and combining are carried out by polarization beam splitters.
Thereby, white-color light from the white-color light source 1001 is split into a first color light path (G) and a second-color light paths (R and B) by the dichroic mirror 1004. And, the polarization direction of B-color light is rotated by 90 degrees by the first color-selective wave plate 1008, wherein the B-color light is made into P-polarized light, and R-color light is made into S-polarized light, and the respective lights are split into a third-color light path (R) and a fourth-color light path by the second polarization beam splitter 1006.
Further, in the first light path, light reflected by the first polarization beam splitter 1005 is further reflected with the polarization direction thereof rotated by 90 degrees by the first reflection type liquid crystal display element 1002G, is transmitted through the first polarization beam splitter 1005, and reaches the projection optical system 1003 after being reflected by the third polarization beam splitter 1007. And, in the third light path, the polarization direction thereof is rotated by 90 degrees by the second reflection type liquid crystal display element 1002R, and the light is transmitted through the second polarization beam splitter 1006. Further, in the fourth light path, the polarization direction is rotated by 90 degrees by the second reflection type liquid crystal display element 1002B. Then, two color lights (R and B) are combined into a single light flux after being reflected by the second polarization beam splitter 1006. And, the polarization direction of B-color light is rotated by 90 degrees by the second color-selective wave plate 1009, and color light of R and B is made into P-polarized light and reaches the projection optical system 1003 after being transmitted through the third polarization beam splitter 1007, wherein a three-color image is combined.
However, since, in the conventional example, a color-selective wave plate having the same characteristics in terms of rotational polarization is used for the first and second color-selective wave plates 1008 and 1009, which are installed at the incidence side and emergence side of the second polarization beam splitter 1006, unnecessary polarized light components occur in a region of transition (hereinafter called a “transition region”) in which the characteristics of rotational polarization is converted from 0 degrees (no rotation is brought about) to 90 degrees.
A detailed description is given of the problem. FIG. 34 expresses characteristics of rotational polarization of a color-selective wave plate used in the prior art example. The color-selective wave plate is featured in that polarization is rotated by 90 degrees in a wavelength region of blue (B) and the polarization is not rotated in a wavelength region of red (R), and the wavelength region between the regions is a transition region. FIG. 34 is a characteristic view showing actions of rotating the polarization of the color-selective wave plate. In another view, the characteristic expresses the ratio of polarized light components orthogonal to incident polarized light components in incident linear polarization (when the ratio is 1, the polarization direction is rotated by 90 degrees). Therefore, in the drawing, a scale expressing the ratio (percentage) of polarized light components orthogonal to the incident polarization component is described on the right side of the graph.
In this case, a description is given of how the incident linear polarization light is converted, based on respective combinations of color-selective wave plates.
A first state shows a case where the polarization direction is rotated by 90 degrees with the first and second color-selective wave plates. When a characteristic for expressing the ratio of polarized light components orthogonal to incident polarization is I(λ), and a characteristic expressed byC1(λ)=I(λ)×I(λ)is C1(λ), it is possible to express the ratio (percentage) of light amount for which polarization direction is rotated by 90 degrees with the first and second color-selective wave plates. CI(λ) is shown in FIG. 35.
A second state is a case where the polarization direction is not rotated with the first and second color-selective wave plates. Where it is assumed that a characteristic expressed byC2(λ)=(1−I(λ))×(1−I(λ))is C2(λ), it is possible to express the ratio of light amount for which the polarization direction is not rotated by the first and second color-selective wave plate. C2(λ) is shown in FIG. 36.
A third state is a case where the polarization direction is rotated by 90 degrees with the first color-selective wave plate and the polarization direction is not rotated with the second color-selective wave plate. Where it is assumed that a characteristic expressed byC3(λ)=I(λ)×(1−I(λ))is C3(λ), it is possible to express the ratio of light amount for which the polarization direction is rotated by 90 degrees by the first color-selective wave plate and the polarization direction is not rotated with the second color-selective wave plate. C3(λ)is shown in FIG. 37.
A fourth state is a case where the polarization direction is not rotated with the first color-selective wave plate and the polarization direction is rotated by 90degrees with the second color-selective wave plate. Where the characteristic expressed byC4(λ)=(1−I(λ))×I(λ)is made into C4(λ), it is possible to express the ratio of the light amount for which the polarization direction is not rotated with the first color-selective wave plate but the polarization direction is rotated by 90 degrees with the second color-selective wave plate. The C4(λ) will have the same characteristic as that of C3(λ).
Although the first state corresponds to the B component and the second state corresponds to the R component, it is found that unnecessary transition characteristics occur in the transition region of the color-selective wave plate in the third and fourth states other than the above.
Next, a description is given of an influence which the unnecessary components in an optical system shown with a conventional example exert on the contrast of an image projection apparatus.
The influence exerted on the contrast is an amount of light leaking into the projection optical system when a reflection type liquid crystal display element is displayed in black. In the third state, light is made incident into the second polarization beam splitter 1006 as a P-polarized component by the first color-selective wave plate, is transmitted through the polarization splitting film and is reflected with the polarization state not changed by the reflection type liquid crystal display element (that is, with the display in black). And, the light is again made incident into the second polarization beam splitter 1006 as a P-polarized component, is reflected by the polarization splitting film, and is transmitted through the third polarization beam splitter 1007, as it is, as the P-polarized component without being subjected to any rotating action of polarization direction by the second color-selective wave plate. Thus progressing light is reflected by the reflection type image display element and is once analyzed when being reflected as the P-polarized light from the polarization splitting film of the second polarization beam splitter 1006. However, it is not analyzed with respect to the third polarization beam splitter 1007 since it is made incident as the P-polarized light in the transmitting polarization direction. Resultantly,in the third state, the leakage light amount is remarkably increased relative to the first state brought about by a normal action.
This cannot be cut even if a polarization plate is provided between the second polarization beam splitter 1006 and the third polarization beam splitter 1007 as shown in the conventional example.
In the fourth state, the light is made incident into the second polarization beam splitter 1006 as S-polarized light, is reflected by the polarization splitting film and is reflected by the reflection type liquid crystal display element with the polarization state not changed (that is, with display in black) and is again incident into the second polarization beam. And, the light is transmitted through the polarization splitting film and is transmitted through the third polarization beam splitter 1007, as it is, as the P-polarized component by being subjected to a rotating action of polarization by the second color-selective wave plate. Thus progressing light is not analyzed since it is made incident into the third polarization beam splitter 1007 as P-polarized light in the transmitting polarization direction. Resultantly,in the fourth state, the leakage light amount will be increased relatve to the second state brought about by a normal action as in the third state.
The conventional example describes that the transition region of the color-selective wave plate is set to a wavelength region cut by a dichroic mirror.
However, since the reflectivity of the dichroic mirror is not 100% and the wavelength characteristics thereof shift in accordance with the incident angle, it is impossible that the light amount in the transition region is made into zero. Therefore, it has been made clear by research that the conventional structure is insufficient as an optical system to secure high-quality contrast.